Digital audio and video products have become very popular because of their many advantages such as high signal fidelity. In the past, digital signals of such digitised audio and video products, such as CDs and LDs, were usually converted into analogue signals before amplification for output. With the increasing availability of audio and visual products in digitised forms, it has been noted that analogue conversion of the digital signals for amplification before output to the appropriate load, for example, speakers or TV monitors, would introduce additional errors and distortions. Hence, it is generally accepted that direct amplification of the digital signals for output to the appropriate loads would be preferable.
In a typical full digital system, pulse-code-modulated (PCM) signals are usually output from a signal content source. The PCM signals are then converted into pulse-width-modulated (PWM) signals for amplification by a PWM amplifier. Compared with analogue amplifiers, digital amplifiers are known to have the advantages of having a compact size, high efficiency, low cross-channel interference and high fidelity. Thus, in a typical digital system, PCM signals are first converted into PWM signals for amplification by a PWM amplifier. However, it is well known that it is difficult to obtain both a high dynamic range and a high resolution in a digital amplification system.
For example, for a 16 bit PCM system at a sampling frequency of 44.1 KHz, a digital signal processor (DSP) will be required to have a clock rate of fclk=216fc=216×44.1=2.89 GHz. When 8 times interpolation is used, the clock requirement on the DSP will then become fclk=8·216fc=8×216×44.1=23.1 GHz. Although such a clock rate may become practical in the future, it is highly desirable and will be more economical if there can be provided means to enhance signal resolution and dynamic range without depending on the availability of a DSP with such a high clock rate. To alleviate the requirements of such a high clock rate for a higher signal resolution, prior art suggested using 8 bit PWM signal conversion of a 16 bit PCM signal so that a DSP clock rate of fclk=8·28fc=8×28×44.1=90.3 MHz, which corresponds to a minimum pulse width of 11 ns, can be used. However, such an approach introduces higher quantization noise and results in loss in fidelity which is contrary to the purpose of using a 16 bit system and also represents a loss in the DSP performance.
For a typical 16 bit PCM system at an original sampling frequency of 44.1 KHz, it is common to increase the sampling frequency to 352.8 KHz by 8 times interpolation so that the PWM frequency is at 352.8 KHz with a maximum PWM pulse width of 1/352.8KHz=2834 ns. Hence, the minimum and maximum pulse width of the PWM pulse are respectively 11 ns and 2834 ns, corresponding to a resolution of 11 ns/2834 ns=3.88×10−3 which is not sufficient for high fidelity acoustic reproduction. Furthermore, as the pulse width variation steps are evenly distributed across the entire pulse width of a typical PWM format, the resolution at the lower end of the input signal amplitude will be limited, thus affecting the resolution and the dynamic range. On the other hand, while the use of a DSP with a higher clock rate will result in a narrower pulse width and therefore higher resolution and lower noise, the resulting problems such as higher costs, electromagnetic interference, problems associated with high speed circuitry design as well as additional pins, make this approach non-attractive. Therefore, it is highly desirable if there can be provided improved means, methods, apparatus and device for enhanced PWM signal amplifications.